<p>Baseline characteristics of the study population.</p>

Circulating immune complexes (CIC) cause different organ lesions in canine leishmaniosis. This study aimed to measure CIC in dogs infected with Leishmania (L.) infantum in a non-endemic country and to analyze associations with disease parameters. Measurement of Leishmania-specific CIC was performed by a polyethylene glycol ELISA (cut-off: 0.274 optical density (OD)) every three months during a one-year study period in 52 L. infantum-infected dogs. Each appointment included a physical examination, complete blood count, serum biochemistry including C-reactive protein, urinalysis, L. infantum PCR and antibody ELISA. Statistical analyses included Mann-Whitney U tests, a multivariable robust linear regression, Spearman correlation, univariable logistic regression, and a receiver operating characteristic (ROC) curve. CIC levels differed significantly between dogs with and without lymphadenopathy (p < 0.01; β=−0.38; 95% CI: −0.52, −0.24), uveitis (p < 0.01; β=−0.52; 95% CI: −0.89, −0.15), seborrhea/hypotrichosis (p = 0.04; β=−0.11; 95% CI: −0.21, −0.01), and skin nodules (p < 0.01; β = 0.22; 95% CI: 0.13, 0.31). Significant moderate to strong correlations with CIC levels were found for L. infantum antibodies (p < 0.01; rs = 0.65; 95% CI: 0.57, 0.72), globulin (p < 0.01; rs = 0.60; 95% CI: 0.51, 0.68), albumin-to-globulin ratio (p < 0.01; rs=−0.56; 95% CI: −0.65, −0.47) and total protein (p < 0.01; rs = 0.46; 95% CI: 0.35, 0.56). CIC levels were significantly higher in dogs with positive lymph node PCR (p = 0.04) as well as in dogs with disease relapses (p < 0.01). The ROC curve analysis revealed one highly specific cut-off value at 1.668 OD for differentiating between dogs with and without disease relapse (98% specificity; 60% sensitivity). Measurement of Leishmania-specific CIC is useful for monitoring dogs with L. infantum infections and for indicating disease relapse.